1. Field of the Invention
The present invention relates to a radar device and a method of calculation of receive power in that radar device, more particularly relates to a radar device which receives radio waves sent from a vehicle and reflected back from a target at a plurality of receive antennas to detect a position of the target and a method of calculation of receive power in that radar device.
2. Description of the Related Art
In the past, there have been radar devices which continuously measure the distance and direction of preceding vehicles or targets in the front (obstacles) or of vehicles or other targets approaching from the rear relative to a vehicle being driven by a user and which prevent collisions or enable automatic driving. Such a radar device sent out transmit waves from an antenna installed in the vehicle, received reflected waves bounced back after hitting a target at another antenna, processed the receive signals, and estimated the direction of arrival the reflected waves to detect the target. As the method for estimating the direction of arrival of reflected waves, the DBF method, Capon method, LP method, minimum norm method, MUSIC method, ESPRIT method, and PRISM method are known.
DBF: Digital Beam Forming
LP: Linear Prediction
MUSIC: Multiple Signal Classification
ESPRIT: Estimation of Signal Parameters via Rotational Invariance Techniques
PRISM: Propagator method based on an Improved Spatial-Smoothing Matrix
In the method of estimation of the direction of arrival of reflected waves, the device sends out transmit waves consisting of radio waves modulated in frequency up and down, receives waves reflected from a target by electronic scan radar using a plurality of receive antennas, estimates the direction of arrival of the target at each peak from the peak frequency signals at the up time and down time of the frequency, and then uses the powers as the basis for pairing to calculate the distance and relative speed. The electronic scan radar described in Japanese Patent No. 4415040 found the vectors of the frequency spectrums which were detected for the antennas in the up section of two frequencies, found the combined value when making the vectors of the spectrums of the different frequencies the same angle, and found the distance etc. of a target from the combined value of the vectors. In high resolution processing, as representative systems comparing the spectrum values with the power, there are the DBF method and the Capon method. For this reason, when finding the power of a direction of arrival, the DBF method or the Capon method has mostly been used.
Further, as the method of calculation of the power of a target, up until now the Capon method has mostly been used. The reason is that in the DBF method, other arrival waves are received at the side lobes, so sometimes the power of the reflected waves could not be accurately calculated. As opposed to this, in the Capon method, by turning the main lobe toward a certain direction and simultaneously minimizing the contribution from other directions to the output, the power of the reflected waves is found more accurately than the DBF method. That is, compared with the DBF method, in the Capon method, the spectrum values are output proportionally by the power of the receive waves, so the power of the reflected waves is accurately calculated. Note that, when estimating the direction of arrival of the reflected waves, usually the spatial average method is used for removing the cross correlation components by preprocessing of angle estimation.
In this regard, when using the Capon method to calculate the power of the reflected waves, if there are a plurality of targets, reflected waves from the plurality of targets end up being received by the antennas and the cross correlation components of the targets are not removed at the time of preprocessing of angle estimation using the spatial average method, so there is the problem that the receive power cannot be accurately calculated. This becomes remarkable when there are a small number of receive antennas or when there are a small number of snapshots.
Further, in an electronic scan radar device using a plurality of receive antennas, the receive power value of the arrival waves is used conditional on pairing of the targets obtained in the up section of the frequency and targets obtained in the down section of the frequency. Therefore, if calculation of the accurate receive power is not possible, the following problems arise.
(1) The above condition of pairing includes the condition of “close power values of angles”. This is no longer satisfied and the possibility of mispairing occurring rises. This ends up causing the electronic scan radar device to malfunction.
(2) In composite processing of high resolution processing and a system which requires precision of the power such as an amplitude monopulse, it is not possible to improve the precision of the amplitude monopulse.